#include<bits/stdc++.h>
using namespace std;
int main(){
	int x1,x2,y1,y2;
	cin>>x1>>y1>>x2>>y2;
	x2=-x2;
	double k=(y2-y1)*1.0/(x2-x1)*1.0;
	double b=y1*1.0-k*x1*1.0;
	cout<<b;
	return 0;
}